During the process of indoor propagation of sound signal, due to the sound reflection caused by hard interfaces such as walls and floors, the sounds reaching the microphone further comprise the sound signals through one or more reflections in addition to the direct sounds directly from the sound source. These non-direct sounds constitute reverberation signals. The sound signals through one or a few reflections are called early reflection signals, which constitute early reverberation signals that can enhance the voice. The sound signals through multiple reflections are called late reflection signals, which constitute late reverberation signals. Strong late reverberation will reduce the intelligibility of the voice.
In some hands-free voice communication, if the caller is far from the microphone, the voice intelligibility will be decreased due to room reverberation, resulting in poor call quality. Thus, some technique is needed to reduce reverberation and improve voice intelligibility. The signals received by a microphone comprise direct sound signals and reverberation signals. According to the foregoing, the reverberation includes early reverberation and late reverberation. It is mainly late reverberation that reduces the voice intelligibility, while early reverberation can generally enhance the voice. Therefore, the key to enhance the intelligibility is to reduce the late reverberation singals.
In various reverberation reduction techniques, the method for eliminating reverberation by spectral subtraction based on double microphones has drawn more attention. In the existing method for eliminating reverberation by spectral subtraction based on double microphones, two channels of signals are obtained using an adaptive beamforming (GSC) structure, wherein the first channel of signals are output of the delay-sum beamformer, and the second channel of signals are output of the blocking matrix. The reverberation of the first channel of signals is estimated by the energy envelopes of the two channels of signals via an adaptive filter, and then the reverberation is removed using a spectral subtraction method. This method has several disadvantages:
1) it will remove the early reverberation, and thus the processed sound will become thin;
2) it does not judge the strength of the reverberation and uses the same spectral subtraction process in different reverberation cases, which may damage the voice quality in the case of weak reverberation and higher original voice intelligibility; and
3) it requires an accurate estimation of the direction of arrival of the direct sound, so as to separate the direct sound, and thus, it requires high consistence of the microphones and strict limits to the acoustic design.